UCD problem

[Regnad]

I would appreciate advice from someone familiar with UCD400 modules. On all 3 modules in my amp, the two 1.1K resistors near the top of the UCD400 board are very hot. I measure 22 volts across them and that, if I'm right, is about 0.4 watts which seems too much.

Is a schematic available?

HG supply with 60 volt rails. Otherwise, the modules work OK.

Thanks a LOT for any help...

[BWRX]

What size resistors are they? Are they large 2512's? If so, they can handle 0.5W and they should be fine. Are they part of the op amp supply circuit?

[Regnad]

Thanks for the reply...

They are leaded metal films and look to me to be about 1/4 watt. Anyway, they are too hot to hold a finger on which is never good. I don't have a schematic to see their purpose.

[BWRX]

If the 400 modules are like the 180 modules those resistors are there to drop some voltage for the op amp power supply transistors. According to your measurements there is about 20mA flowing through each resistor. Some of that goes to bias the zeners and the rest goes through the transistor base and collector. I don't know what is causing that much current draw, but it's either the op amp circuit or the zeners are being biased with a lot of current. You'd need to trace out the circuit to see exactly how everything is connected.

Assuming the zeners used are 15V types and 0.7Vbe drop across the transistors;
60-22=38V
38-14.3=23.7Vce across the transistor
Pdiss=(20mA)*(23.7V)=0.474Wmaximum dissipated by the transistors assuming little is used to bias the zeners and little is used for base current. These must be SOT223 or TO220 types to safely handle that kind of dissipation.

Just thought of something, do you know if your modules have the op amp biased into class A? I remember reading about the option to do this on some UcD modules.

[Regnad]

The modules are the latest stock version with the AD8620 option.

The resistors are connected to the main voltage rails. I will take an older module to work tomorrow and trace some of the circuitry to see if I can figure anything out.

I do seem to remember seeing a schematic somewhere.

[Jan-Peter]

These resistors are in serie with the 12V on board regualtor.

The resistor is a 0.6W rated at 70oC metal film resistor.

Nothing to worry about......

Cheers,

Jan-Peter

[lucpes]

Kinda off-topic... would it help to remove these resistors and connect a clean regulated supply here say +/-24V to feed the onboard regulators or is it better to remove the transistors and directly feed the front-end with +/- 12V?

[Jan-Peter]

No difference....

Jan-Peter

[Regnad]

Thanks a lot for the explanation and reassurance...

[yojiunn]

Recently, I use 2 UCD400s with SPS80, I got a problem, I check the SPS80 Voltage(without UCD400 connected), it is +(-)59.5V, but when I connect 2 UCD400, the voltage become +64V, -55V. UCD400 still work well, the music was well. Is that any problem? Thanks.